In 3rd Generation Partnership Project Radio Access Network Long Term Evolution (3GPP-LTE (hereinafter, referred to as LTE)), Orthogonal Frequency Division Multiple Access (OFDMA) is adopted as a downlink communication scheme, and Single Carrier Frequency Division Multiple Access (SC-FDMA) is adopted as an uplink communication scheme (e.g., see NPL-1, NPL-2, and NPL-3).
In LTE, a base station apparatus for radio communications (hereinafter, abbreviated as “base station”) performs communications by allocating a resource block (RB) in a system band to a terminal apparatus for radio communications (hereinafter, abbreviated as “terminal”) for every time unit called “subframe.”
The base station also transmits allocation control information (i.e., L1/L2 control information) for the notification of the result of resource allocation of downlink data and uplink data to the terminal. As this allocation control information, Downlink Control Information (DCI) which is downlink allocation control information is transmitted. There are two types of DCI (which will be described later); common DCI targeting all terminals and specific DCI targeting a specific terminal (specific terminal or terminal in a specific group).
Furthermore, control information such as DCI is transmitted to a terminal using a downlink control channel such as a Physical Downlink Control Channel (PDCCH). Here, the base station controls the resource amount, that is, the number of OFDM symbols, used for transmission of a PDCCH in subframe units according to the number of terminals allocated or the like. To be more specific, the resource amount used for transmission of a PDCCH is set variably over the entire system band in the frequency-domain, and three OFDM symbols from a leading OFDM symbol to a third OFDM symbol of one subframe in the time-domain. The base station reports to the terminal, a Control Format Indicator (CFI) which is information indicating the number of OFDM symbols available for transmission of a PDCCH with the leading OFDM symbol of each subframe using a Physical Control Format Indicator Channel (PCFICH). The terminal receives DCI according to the CFI detected from the received PCFICH. Furthermore, the base station transmits a HARQ Indicator (HI) indicating delivery acknowledgment information (ACK/NACK) for uplink data to the terminal using a Physical Hybrid ARQ Indicator CHannel (PHICH) (e.g., see NPL-1). In LTE a frequency band having a system bandwidth of up to 20 MHz is supported.
Each PDCCH also occupies a resource composed of one or more consecutive control channel elements (CCEs). A CCE is a minimum unit of radio resource allocated to a PDCCH. Furthermore, a CCE is made up of a plurality of consecutive resource element groups (REGs) composed of resource elements (REs). For example, one REG is made up of four REs. To be more specific, a CCE is made up of a plurality of consecutive REGs (e.g., nine consecutive REGs) among REGs not allocated as radio resources for the aforementioned PCFICH and PHICH. Furthermore, the base station may also perform interleaving processing in REG units with resources for a PDCCH for each terminal in order to randomize interference.
In LTE, the number of CCEs occupied by a PDCCH (the number of concatenated CCEs: CCE aggregation level) is selected from 1, 2, 4, and 8 depending on the number of information bits of allocation control information or the condition of a propagation path of a terminal. At this time, allocatable CCEs are predetermined for each CCE aggregation level (e.g., see PTL 1). For example, when the CCE aggregation level is n (e.g., n=1, 2, 4, 8), the base station can allocate to a PDCCH for the terminal, only n consecutive CCEs starting from a CCE with a CCE index (CCE number) corresponding to a multiple of n. On the other hand, since the terminal cannot know which CCE is allocated to a PDCCH for the terminal and what the CCE aggregation level is, the terminal has to try decoding (blind decoding) on a PDCCH for all CCEs which may be allocated to the PDCCH for the terminal using a round-robin method. For this reason, as described above, it is possible to reduce the number of trials of PDCCH decoding at the terminal by providing constraints (tree-based structure) of CCEs allocatable to the PDCCH.
Furthermore, if a base station allocates a plurality of terminals to one subframe, the base station transmits a plurality of items of DCI via a plurality of PDCCHs at a time. In this case, in order to identify a terminal to which each PDCCH is transmitted, the base station transmits the PDCCH with CRC bits included therein, the bits being masked (or scrambled) with a terminal ID of the transmission destination terminal. Then, the terminal performs demasking (or descrambling) on the CRC bits of a plurality of PDCCHs for the terminal with its own ID, thereby trial-decoding (hereinafter, referred to as “blind-decoding”) the DCI to detect the DCI for the terminal.
Also, for the purpose of reducing the number of DCI blind decoding operations on a terminal, a method for limiting CCEs targeted for blind decoding for each terminal is under study. This method limits a CCE region that may be targeted for blind decoding by each terminal (hereinafter, referred to as “search space (SS)”). There are two types of search space; common search space (hereinafter, referred to as “C-SS”) and terminal (UE) specific search space (or UE specific by C-RNTI Search Space: hereinafter, referred to as “UE-SS”). The terminal performs blind decoding on DCI in a C-SS and DCI in a UE-SS corresponding to the terminal.
A C-SS is a search space common to all the terminals, indicating a range of CCEs in which all the terminals perform blind decoding on DCI. A C-SS is allocated with a PDCCH which is simultaneously reported to a plurality of terminals for transmitting control information for data allocation common to terminals (e.g., dynamic broadcast channel (D-BCH), paging channel (PCH) and RACH response or the like) (hereinafter, referred to as “allocation control information for a common channel”). A C-SS includes six candidates targeted for blind decoding in total, namely, 4 candidates (=16 CCEs (=4 CCEs×4 candidates)) and 2 candidates (=16 CCEs (8 CCEs×2 candidates)) with respect to the CCE aggregation level, 4 and 8, respectively.
On the other hand, a UE-SS is a search space specific to each terminal and is randomly configured for each terminal. For example, a UE-SS in each terminal is configured using a terminal ID of each terminal and a hash function which is a function for randomization. The number of CCEs that forms this UE-SS is defined based on the CCE aggregation level of a PDCCH. For example, the number of CCEs forming search spaces is 6, 12, 8, and 16 in association with CCE aggregation levels of PDCCHs 1, 2, 4, and 8 respectively. That is, the number of blind decoding region candidates is 6 (6 CCEs (=1 CCE×6 candidates)), 6 (12 CCEs (=2 CCEs×6 candidates)), 2 (8 CCEs (4 CCEs×2 candidates)), and 2 (16 CCEs (=8 CCEs×2 candidates)) in association with CCE aggregation levels of PDCCHs 1, 2, 4, and 8 respectively. That is, blind decoding region candidates are limited to 16 candidates in total. For example, a UE-SS is allocated with a PDCCH for transmitting uplink scheduling information and downlink scheduling information directed to the target terminal.
Thus, each terminal needs only to perform blind decoding on only a group of blind decoding region candidates in search spaces (C-SS and UE-SS) allocated to the terminal in each subframe, allowing the number of blind decoding operations to be reduced.
Here, a C-SS and a UE-SS may be configured so as to overlap each other, or UE-SSs may also be configured so as to overlap each other. However, when UE-SSs for a plurality of terminals overlap each other, a case may be assumed where the base station cannot allocate CCEs to a PDCCH directed to a specific terminal. Thus, the probability that the base station is not allowed to allocate CCEs to a PDCCH is referred to as “blocking probability.”
For example, a case will be described where 32 CCEs of CCE0 to CCE31 (CCE numbers 0 to 31) are defined. In this case, the base station sequentially allocates CCEs to a PDCCH for each terminal.
Here, suppose, for example, CCE2 to CCE9, and CCE13 to CCE19 have already been allocated to a PDCCH. In this case, when a UE-SS corresponding to the next PDCCH (CCE aggregation level=1) is configured of CCE4 to CCE9, the base station cannot allocate CCEs to this PDCCH because CCE4 to CCE9 (all CCEs in the UE-SS) are already allocated to the other PDCCH.
Furthermore, suppose, in another example, CCE0, CCE1, CCE6 to CCE9 and CCE13 to CCE19 are already allocated to a PDCCH. In this case, when a UE-SS corresponding to the next PDCCH (CCE aggregation level-4) is configured of CCE0 to CCE7, the base station cannot allocate CCEs to this PDCCH (CCE aggregation level=4). This is because the CCE aggregation level is based on a tree-based structure, and so the base station can allocate only 4 CCEs of CCE0 to CCE3 or 4 CCEs of CCE4 to CCE7 to this PDCCH (CCE aggregation level=4). That is, CCE0 and CCE1 among 4 CCEs of CCE0 to CCE3 are already allocated to the other PDCCH, and CCE6 and CCE7 among 4 CCEs of CCE4 to CCE7 are already allocated to the other PDCCH.
Thus, when the base station fails to allocate CCEs to the PDCCH for the terminal, the base station changes the CCE aggregation level and allocates a plurality of consecutive CCEs to the PDCCH for the terminal based on the changed CCE aggregation level.
For example, as in the case of the aforementioned example, suppose CCE2 to CCE9, and CCE13 to CCE19 are already allocated. In this case, when a UE-SS corresponding to the next PDCCH (CCE aggregation level=1) is CCE4 to CCE9, CCEs cannot be allocated to this PDCCH as described above. In this case, the base station changes the CCE aggregation level from 1 to 2. This causes the UE-SS corresponding to the PDCCH (CCE aggregation level=2) to be changed from CCE4 to CCE9 (6 CCEs) to CCE8 to CCE19 (12 CCEs). As a result, the base station can allocate CCE10 and CCE11 to this PDCCH (CCE aggregation level=2). Even when the CCE aggregation level is changed in this way, if CCEs cannot yet be allocated to the PDCCH, the base station attempts transmission or the like in the next subframe.
Furthermore, downlink control information transmitted from the base station is called “DCI” as described above, and contains information on resource allocated to the terminal by the base station (resource allocation information), and modulation and channel coding scheme (MCS). The DCI has a plurality of formats. That is, examples thereof include an uplink format, downlink multiple input multiple output (MIMO) transmission format, and downlink non-consecutive band allocation format. The terminal needs to receive both downlink allocation control information (downlink-related allocation control information) and uplink allocation control information (uplink-related allocation control information).
For example, for the downlink control information (DCI), formats of a plurality of sizes are defined depending on a method for controlling a transmission antenna of a base station and a method for allocating a resource. Among the formats, a downlink allocation control information format for consecutive band allocation (hereinafter, simply referred to as “downlink allocation control information”) and an uplink allocation control information format for consecutive band allocation (hereinafter, simply referred to as “uplink allocation control information”) have the same size. These formats (i.e., DCI formats) include type information (for example, a one-bit flag) indicating the type of allocation control information (downlink allocation control information or uplink allocation control information). Thus, even if DCI indicating downlink allocation control information and DCI indicating uplink allocation control information have the same size, a terminal can determine whether specific DCI indicates downlink allocation control information or uplink allocation control information by checking type information included in allocation control information.
For example, the DCI format in which uplink allocation control information for consecutive band allocation is transmitted is referred to as “DCI format 0” (hereinafter, referred to as “DCI 0”), and the DCI format in which downlink allocation control information for consecutive band allocation is transmitted is referred to as “DCI format 1A” (hereinafter, referred to as “DCI 1A”). DCI 0 and DCI 1A are of the same size and distinguishable from each other by referring to type information as described above. Hereinafter, DCI 0 and DCI 1A will be collectively referred to as DCI 0/1A.
In addition to these DCI formats, there are other formats for downlink, such as DCI format used for common channel allocation (DCI format 1C: hereinafter, referred to as “DCI 1C”), DCI format used for non-consecutive band allocation on a downlink (DCI format 1: hereinafter, referred to as “DCI 1”) and DCI format used for allocating spatial multiplexing MIMO transmission (DCI formats 2, 2A, 2B and 2C: hereinafter, referred to as “DCI 2, 2A, 2B and 2C”). Furthermore, there are also other DCI formats, such as DCI formats 1B and 1D (hereinafter, referred to as “DCI 1B and 1D”). Here, DCI 1, 1B, 1D, 2, 2A, 2B and 2C are formats used depending on the downlink transmission mode of the terminal (format of specific DCI). That is, DCI 1, 1B, 1D, 2, 2A, 2B and 2C are formats configured for each terminal. On the other hand, DCI 0/1A is a format independent of the transmission mode and available to a terminal in any transmission mode. That is, DCI 0/1A is a format commonly used for all the terminals (common DCI format). If DCI 0/1A is used, single-antenna transmission or a transmission diversity scheme is used as a default transmission mode. In the above description, specific DCI formats (DCI 2, 2A, 2B and 2C) in which spatial multiplexing MIMO transmission corresponding to a plurality of layers may also be generically called “DCI family 2.” Furthermore, specific DCI formats (DCI 1, 1B and 1D) corresponding to a single layer may also be generically called “DCI family 1.” A correlation between a DCI format and a transmission mode is defined (e.g., see Table 7-1-5 of NPL-4).
Here, DCI 1A used for common channel allocation and DCI 0/1A used for terminal-specific data allocation have the same size, and terminal IDs are used to distinguish between DCI 1A and DCI 0/1A. To be more specific, the base station applies CRC masking to DCI 1A used for common channel allocation so as to be distinguishable with a terminal ID common to all the terminals. Furthermore, the base station applies CRC masking to DCI 0/1A used for terminal-specific data allocation so as to be distinguishable with a terminal ID assigned in a terminal-specific manner. Therefore, the base station can transmit DCI 0/1A used for terminal-specific data allocation also using a C-SS without increasing the number of blind decoding operations of the terminal.
Also, the standardization of 3GPP LTE-Advanced (hereinafter, referred to as “LTE-A”), which provides a data transfer rate higher than that of LTE, has been started. In LTE-A, a downlink transfer rate of maximum 1 Gbps or higher and an uplink transfer rate of maximum 500 Mbps or higher are achieved. Therefore, a base station and a terminal capable of communicating at a wideband frequency of 40 MHz or higher (hereinafter, referred to as “LTE-A terminal”) will be introduced. An LTE-A system is also required to support a terminal designed for an LTE system (hereinafter, referred to as “LTE terminal”) in addition to an LTE-A terminal.
In LTE-A, a transmission method using non-consecutive band allocation and a transmission method using MIMO will be introduced as new uplink transmission methods. Accordingly, the definitions of new DCI formats (e.g., DCI formats 0A, 0B and 4: hereinafter, referred to as DCI 0A, 0B and 4)) are being studied (e.g., see NPL-4 and NPL-5). In other words, DCI 0A, 0B and 4 are DCI formats dependent on an uplink transmission mode.
As described above, in LTE-A, if a DCI format dependent on a downlink transmission mode (one of DCI 1, 1B, 1D, 2, 2A, 2B and 2C), a DCI format dependent on an uplink transmission mode (one of DCI 0A, 0B and 4), and a DCI format independent of a transmission mode and common to all the terminals (DCI 0/1A) are used in a UE-SS, then the terminal performs blind decoding (monitoring) on PDCCHs of the abovementioned three DCI formats respectively. For example, since 16 blind decoding operations (blind decoding region candidates: 16 candidates in total) in one DCI format need to be performed, 48 (=16×3) blind decoding operations in total are performed.
Furthermore, in LTE-A, if DCI 1C and DCI 1A which are common channel allocation formats are used in a C-SS, then the terminal performs blind decoding (monitoring) on PDCCHs of the abovementioned two DCI formats. For example, since 6 blind decoding operations (blind decoding region candidates: 6 candidates in total) in one DCI format need to be performed in a C-SS, 12 (=6×2) blind decoding operations in total are performed. Therefore, the terminal performs 60 (=48+12) blind decoding operations in total per subframe.
Additionally, in LTE-A, to achieve an increased coverage, the introduction of radio communication relay apparatus (hereinafter, referred to as “relay station”) has been specified. Accordingly, the standardization of downlink control channels from base stations to relay stations (hereinafter, referred to as “R-PDCCH (Relay-Physical Downlink Control CHannel)”) is under way (e.g., see NPLs-6, 7, 8 and 9). As a resource region to which an R-PDCCH is mapped (hereinafter, referred to as “R-PDCCH region”), a resource region to which downlink data is mapped (hereinafter, referred to as “PDSCH (Physical Downlink Shared CHannel) region”) is used.
At present, the following matters are being studied in relation to the R-PDCCH.
(1) A mapping start position in the time-domain of an R-PDCCH is fixed to a fourth OFDM symbol from a leading symbol of one subframe, and thus does not depend on the rate at which a PDCCH occupies OFDM symbols in the time-domain.
(2) As a mapping method in the frequency-domain of an R-PDCCH, two disposing methods, “localized” and “distributed” are supported.
(3) As reference signals for demodulation, Common Reference Signal (CRS) and Demodulation Reference Signal (DM-RS) are supported. The base station notifies the relay station of which one of the reference signals is used.
(4) Each R-PDCCH is divided into slot 0 (slot 0 or first slot) and slot 1 (slot 1 or second slot) in one subframe in the time-domain.
(5) Each R-PDCCH occupies a resource configured of one or a plurality of consecutive Relay-Control Channel Elements (R-CCEs).
(6) A PDCCH notifying downlink resource allocation (hereinafter referred to as “DL grant”) is transmitted using slot 0 and a PDCCH notifying uplink resource allocation (hereinafter referred to as “UL grant”) is transmitted using slot 1.
(7) When a data signal (hereinafter, referred to as “PDSCH”) is indicated by an R-PDCCH, a PDSCH is transmitted using only slot 1 or both slot 0 and slot 1 (that is, data transmission using only slot 0 is not possible).